1. Field of the Invention
The present invention generally relates to a developing unit and an image forming apparatus including the same, and more particularly to a developing unit and an image forming apparatus including the same capable of forming a toner image with a stable density.
2. Discussion of the Background
In general, an image forming apparatus employing an electrophotographic method, such as a copying machine, a printer, a facsimile machine, etc. forms an electrostatic latent image on an image carrier. Such an electrostatic latent image is developed into a toner image by a developing unit. The toner image is thermally fixed with a fixing unit and transferred onto a sheet.
In developing units, a two-component developer including a toner and a carrier is widely used. The toner is charged by friction between the carrier and toner, and adheres to the electrostatic latent image due to electrostatic effects. The developer used in a developing process is collected. As toner density in the used developer is decreased, unused toner is added. The unused toner is mixed with the used developer and supplied to an electrostatic latent image through a developing roller.
It is necessary to maintain a certain level of toner density and charge quantity in the developer for a stable toner image. Toner density is determined by toner consumption in the developing process and distribution of supplied toner. The charge quantity of toner is determined by frictional charging during mixing of the carrier and the toner. It is necessary to adequately agitate such a two-component developer.
A related-art developing unit employs a biaxial conveyance method (biaxial developing unit) and includes a developing roller, a transport screw, and an agitation screw. The two screws are horizontally provided side by side beneath the developer carrier. The transfer screw supplies a developer to the developing roller and collects a used developer from the developing roller. The agitation screw transports the developer in a direction opposite to a flow direction of the developer transported by the supplying and collecting screw.
Another related-art developing unit employs a unidirectional circulation method. A developing unit 101 includes a developing roller 102, a supply passage 103, a collecting passage 104, and an agitating passage 105 provided in parallel with each other as illustrated in FIG. 1. The collecting passage 104 and agitating passage 105 are arranged side by side below the developing roller 102, and the supply passage 103 is arranged above the agitating passage 105. Each of the supply passage 103, collecting passage 104, and agitating passage 105 includes a screw (103a, 104a, and 105a respectively) to transport and/or agitate the developer. Although walls separate the three parts, openings are provided on the walls so that the developer may be unidirectionally circulated in the developing unit 101.
In another related-art unidirectional circulation developing unit, quantities of developer transported by a developer carrier, a collecting screw, an agitation screw, and/or a transporting screw are predetermined to smooth a circulation of developer and to equalize toner density. The collecting screw is capable of transporting more developer than a developer transported by the developer carrier, and the agitation screw or transporting screw is capable of transporting more developer than a quantity transported by the collecting screw.
Toner density may be more stabilized in the unidirectional circulation method than in the biaxial conveyance method. However, the developer may be stressed during exchange between screws, especially when the developer is lifted from a collecting or agitation part located below a developing roller to a supply part located above the developing unit. Further, the charge quantity of the developer may be decreased due to time degradation.